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    <p>第一个</p>
    <img src="css.jpg" width="900" height="400">
   <h1>这是标题</h1>
<p class="tre">To stop the degradation of the planet's natural environment and
     to build a future in which humans live in harmony with nature, by; 
     conserving the world's biological diversity, 
     ensuring that the use of renewable natural resources is sustainable, 
     and promoting the reduction of pollution and wasteful consumption.</p>
<p></p>
<p>第三个段落</p>
<p>第四个段落</p>
<div>
    <p>这个段落是其父元素（div）的首个子元素。</p>
    <p>这个段落不是其父元素的首个子元素。</p>
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    <input type="checkbox" id="myc"/>复选框<br>
    <button onclick="openFullscreen();">打开全屏模式</button><br>
<button onclick="closeFullscreen();">关闭全屏模式</button><br>
<input type="number" min="5" max="9" value="7">
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    checkedb.indeterminate=true;
// 使用 JavaScript 在全屏模式中打开页面
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